Ambulance

Clinical Focus on Emergency Artificial Ventilation
the development of negative intrathoracic
pressure between compressions.”
Reduced pre-load results in lower stroke
volume and, ultimately, decreased cardiac
output.
Emergency ventilation using portable
automatic ventilators
The alternative to using a BV device in
emergency is the portable automatic
ventilator. These are widely used in some
areas of the world and have a number
of advantages over bag-valve devices.
The International Liaison Committee on
Resuscitation ( ILCOR ) reviewed the
relatively limited literature about automatic
ventilators in 2010 2 and produced the
following observations.
1 Automatic ventilators or resuscitators
provide a constant flow of gas to the patient
during inspiration; the volume delivered
is dependent on the inspiratory time (a
longer time provides a greater tidal volume).
Because pressure in the airway rises
during inspiration, these devices are often
pressure limited to protect the lungs against
barotrauma.
2 An automatic ventilator can be used with
either a facemask or other airway device
(e.g., tracheal tube, supraglottic airway
device).
3 An automatic resuscitator should be set
initially to deliver a tidal volume of 6–7 ml
kg−1 at 10 breaths min−1. Some ventilators
have co-ordinated markings on the controls
to facilitate easy and rapid adjustment
for patients of different sizes, and others
are capable of sophisticated variation in
respiratory parameters. In the presence
of a spontaneous circulation, the correct
setting will be determined by analysis of the
patient’s arterial blood gases.
4 Automatic resuscitators provide many
advantages over alternative methods of
ventilation.
• In unintubated patients, the rescuer
has both hands free for mask and airway
alignment.
• Cricoid pressure can be applied with one
hand while the other seals the mask on the
face.
• In intubated patients they free the rescuer
for other tasks.
• Once set, they provide a constant tidal
volume, respiratory rate and minute
ventilation; thus, they may help to avoid
excessive ventilation.
• They are associated with lower peak
airway pressures than manual ventilation,
which reduces intrathoracic pressure and
facilitates improved venous return and
subsequent cardiac output.
A manikin study of simulated cardiac arrest
and a study involving fire-fighters ventilating
the lungs of anaesthetised patients both
showed a significant decrease in gastric
inflation with manually triggered flow-limited
oxygen-powered resuscitators and mask
compared with a BVM 16,17
Figure 3 shows a modern resuscitation
ventilator which is currently used by a
number of emergency medical services.
Figure 3. The Pneupac VR1 resuscitation
ventilator. This device can replace the BVM
and can deliver single manually – controlled
breaths during cardiopulmonary resuscitation
or continuous automatic ventilation.
(Photograph by courtesy of Smiths Medical International
(Luton, UK)
The need for more studies on
automatic ventilators
It is worth noting that the 2010 ILCOR
guidelines cited 23 papers which studied
ventilation as opposed to more than
93 papers concerned with airway
management. Of the ventilation papers
cited, only 3 directly concerned the use
of automatic ventilators. The reason for
there being so few studies on manual and
automatic ventilation is unclear but may be
as a result of ventilation being seen as an
‘obvious’ technique which does not require
investigation. In this respect it is similar to
suction aspiration which is an essential part
of clearing the airway of secretions and
vomitus but which appears never to have
been the subject of a controlled trial.
The evidence available shows that the use
of bag-valve devices is associated with
hyperventilation, in terms of both frequency
and delivered tidal and minute volumes as
well as the peak airway pressure delivered.
The consequences of this in terms of
possible gastric insufflation and barotrauma
are understood. The potential dangers
from volutrauma however has received less
attention, although the damage caused to
the lung parenchyma and subsequent acute
respiratory distress syndrome (ARDS) in the
intensive care setting have been recognized
for many years and have led to the use of
smaller tidal volumes with PEEP to keep
the alveoli open, (the ‘open lung’ strategy
first described by Lachmann 18 ). Given the
potential vulnerability of the lungs and other
organs in a patient with major trauma and
shock, the question of induced volutrauma
in emergency ventilation deserves greater
study.
Bag – valve ventilation compared
with automatic ventilation
There have been some studies which
compare the quality of ventilation delivered
by portable automatic ventilators (known
as ‘automatic transport ventilators’ in US
despite the fact that they are widely used
in emergency ventilation as well as in the
transport of a ventilator-dependent patient
from one location to another).
Salas et al 12 found no differences in delivered
tidal volume between a bag, a valve device
and an automatic ventilator (Impact 730)
using a facemask in a model of adult cardiac
arrest. However, as noted above, they also
found less gastric insufflation and a reduced
mask leak using the ventilator. Their overall
conclusion was ‘that compared with the
BVM the ventilator is at least as effective, is
easier to use, and limits gastric insufflation.
Weiss et al 19 in a study of paramedical
personnel using a BVM or an automatic
ventilator in cardiac arrest concluded that
they were able to accomplish more tasks
and provide better patient care when using
the automatic ventilator. Goedeke et al 20
compared ventilation with a bag valve device
and the Oxylator ventilator. They found the
the bag valve device delivered higher peak
airway pressures and was associated with a
lower SaO 2. They found no differences in
the tidal volumes delivered.
In another study comparing the Oxylator
with bag valve ventilation Noordergraaf
et al 21 found that the bag valve device was
associated with better airway management
and that the Oxylator in automatic
mode delivered hyperventilation. They
recommended that the ventilator should
only be used in manual mode during
resuscitation.
56 Spring 2016 | Ambulancetoday